The present disclosure relates to, for example, demodulators for use in analog or digital radio communication. The present disclosure relates particularly to a system for determining and correcting an amplitude error or an orthogonality error between an in-phase component and a quadrature component of a complex signal after quadrature detection.
In order to receive and demodulate signals modulated by various analog or digital modulation techniques, quadrature detection is typically performed on received signals with a quadrature detector constituted by an analog circuit, and the detection result is obtained as a complex signal (an IQ signal).
An in-phase component and a quadrature component of a complex signal output from a quadrature detector are ideally orthogonal to each other and have the same average amplitude (see FIG. 2A). However, since a quadrature detector constituted by an analog circuit is affected by, for example, characteristic variations among analog devices and temperature characteristics and voltage characteristics of the analog devices, an amplitude error or an orthogonality error occurs between the in-phase component and the quadrature component of the complex signal (see, for example, FIG. 2B). Thus, in signal detection using a complex signal from such a quadrature detector without any additional processing, an image interference occurs to degrade reception performance of a receiver.
To prevent this interference, International Patent Publication No. WO2006/132118 proposes an amplitude error compensator and an orthogonality error compensator that accurately and stably correct an amplitude error and an orthogonality error even with an uneven distribution of constellation points of an input complex signal.